[PATCH v3 3/5] drm/dp: Disable unsupported features in DP_EDP_BACKLIGHT_MODE_SET_REGISTER

[Lyude Paul <lyude@redhat.com>]

As it turns out, apparently some machines will actually leave additional
backlight functionality like dynamic backlight control on before the OS
loads. Currently we don't take care to disable unsupported features when
writing back the backlight mode, which can lead to some rather strange
looking behavior when adjusting the backlight.

So, let's fix this by ensuring we only keep supported features enabled for
panel backlights - which should fix some of the issues we were seeing from
this on fi-bdw-samus.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into DRM helpers")
---
 drivers/gpu/drm/drm_dp_helper.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 4d0d1e8e51fa..d9a7f07f42fd 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -3255,7 +3255,9 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 		return ret < 0 ? ret : -EIO;
 	}
 
-	new_dpcd_buf = dpcd_buf;
+	/* Disable any backlight functionality we don't support that might be on */
+	new_dpcd_buf = dpcd_buf & (DP_EDP_BACKLIGHT_CONTROL_MODE_MASK |
+				   DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE);
 
 	if ((dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) != DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
 		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
@@ -3277,6 +3279,8 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 				    aux->name, ret);
 		else
 			new_dpcd_buf |= DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
+	} else {
+		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
 	}
 
 	if (new_dpcd_buf != dpcd_buf) {
-- 
2.31.1